I found a photo of my Savage on the 40x48 tailer with the center ramp.  It has the 8 inch tires.....and it was towed at 70 mph no problem.  The smaller trailer needs the center ramp to be long enough to haul the bike - but it is cheaper than the folding trailer and it stores up against the wall really easy.

I do know that the Harbor Freight tires have a sticker.......I betcha that is a liability related thing........or a quality issue.  Good quality 8 inch tires can go 70 all day long as long as you keep the pressure up near the rated psi......low pressure allows the sidewalls to flex and it builds up excess heat.